Filters which use active carbon to trap the substances imparting noxious odors to air or which can be considered as contaminants of air, are provided for a wide variety of purposes. For example, they may be used at the discharge side of blowers or the like drawing air from paint-spray booths or regions in which odors are generated. They may be employed wherever hydrocarbons are released into the atmosphere to trap these hydrocarbons at least in part before the air is discharged into the environment.
They may be used upstream of workplaces or other populated locations to protect the population from contaminants which might otherwise be present in the incoming air and, generally can be utilized wherever it is required, desirable or advantageous to remove adsorbable or catalytically modifiable contaminants of toxic, noxious or simply unwanted nature.
In the past, such filters have utilized a filling in the form of a pile of active carbon granules. When the absorption capacity of the active carbon is diminished, the filling must be replaced.
The used active carbon can be destroyed by a thermal treatment at a temperature in excess of 1200.degree. C. at comparatively high cost. It can also be desorbed by the use of steam or other conventional methods. However desorption results in the production of liquid, gaseous or vapor-like materials which can also pose disposal problems.
It has been proposed, in parallel with absorption or adsorption on active carbon, to effect a biochemical modification of the contaminants on the active carbon so as to reduce the rate at which the active carbon might lose its capacity to take up the contaminants. For this purpose, the active carbon may be seeded with appropriate microorganism colonies or populations which can break down the deposited organic contaminants entrained onto the active carbon by the air and can also break down contaminants which remain entrained in the air, these contaminants serving as nutrients for the respective cultures. Thus removal of contaminants is enhanced by a direct action of the microorganism colonies upon nondeposited nutrients and accumulation of contaminants is prevented by the breakdown of deposited contaminants.
In such systems, however, there is a problem with respect to controlling the important parameters like nutrient availability, namely, organic components entrained with the exhaust air, oxygen supply and moisture supply so that an appropriate equilibrium is maintained which can sustain the biological activity on the one hand, but on the other hand will not produce such a growth of the cultures or populations of microorganisms that the interstices between the active carbon grains are obstructed.
Obviously, in most cases, it is difficult, if not impossible to control the air contamination and moisture levels to ensure that uncontrolled growth of the microorganism cultures into the interstices between the granules will not occur.